Ferromagnetic metal particles have found increasing utility in the production of advanced magnetic recording media. These magnetic "pigments" have superior magnetic characteristics, such as coercivity and magnetic remanence, relative to traditional iron oxide particles, which allows their use in high density magnetic media for the recording of video and digital signals. Despite such advantages, however, metal pigments suffer from at least two serious drawbacks: they are hard to disperse in a magnetic medium and they are extremely reactive, even to the point of being pyrophoric. The high reactivity of metal pigments makes their handling and storage quite difficult and had spurred those skilled in the art to develop improved methods for passivating the particles. Thus, for example, a widely practiced method for passivating iron pigment comprises oxidizing its surface in a controlled fashion, thereby imparting a measure of stability to the pigment. Such treatments ar still inadequate since subsequent diffusion of iron atoms leads to a regeneration of a reactive surface upon storage of this stabilized pigment. Moreover, a significant portion of the metallic iron is converted to the oxide by this process, thereby reducing the magnetic properties of the particles and negating some of the advantages of employing metal pigment in the first place. This is particularly true for the case of very small metal particles.
The reactivity of metal pigment, even when partially stabilized by controlled surface oxidation as described above, results in poor storage stability of magnetic recording media made from these particles, particularly under conditions of elevated temperature and humidity. In an effort to further stabilize a metal pigment, Tadokoro et al., in U.S. Pat. No. 4,069,073, disclose a conventional process for the production of a ferromagnetic metal powder comprising reducing a metal salt, wherein the product is further treated with a solution containing phosphate ions. The additional treatment is said to provide metal particles which exhibit humidity resistance.
Various silicone compounds have also found utility in connection with magnetic media based on metallic pigment particles. Thus, for example, in U.S. Pat. No. 4,336,310, Okuyama et al. teach a magnetic recording medium wherein the metal pigment supporting silica on its surface is coated with sequential layers of a hydrolyzed silane and oleic acid. This magnetic medium is said to posses excellent oxidation resistance.
Biermann et al., in U.S. Pat. No. 4,325,739, disclose a pigment of a magnetic metal or alloy coated with at least one composition selected from an ortho-silicic acid ester, a hydrolyzate thereof or a condensation product thereof. The resultant pigment is said to be non-pyrophoric and of high magnetic performance.
In U.S. Pat. No. 4,437,882 to Umemura et al., there is disclosed a nickel-containing iron pigment which is treated with an organic silane compound such as a silicone oil, a silane coupling agent and a silicate. The resulting ferromagnetic powder is said to have excellent oxidative stability and a high saturation magnetization.
Another pigment which is coated with a silane is taught by Matsufuji et al. in U.S. Pat. No. 4,475,946. The metal particles are said to be readily dispersible in a magnetic recording medium and to have excellent stability and corrosion resistance.
The combination of a phosphate ester and silicone compound for use in magnetic media is disclosed by Sylvester et al. in U.S. Pat. No. 4,640,790, assigned to the assignee of the present invention. In this patent, a reaction product of the phosphate ester and an alkali siliconate silylalkylphosphonate is shown to act as an improved dispersant for magnetic particles.